1. Field of the Invention
The present invention relates to a method of feeding silicon material at the time of manufacture of a silicon monocrystal using the Czochralski method as well as to a feed pipe used in the method . Further, the present invention relates to a method of manufacturing a silicon monocrystal using the Czochralski method.
2. Description of the Related Art
A multi-pulling method has been known as a method of reducing costs associated with manufacture of a silicon monocrystal using the Czochralski method (see "SEMICONDUCTOR SILICON CRYSTAL TECHNOLOGY," Fumio SHIMURA, pp. 178-179, 1989).
According to the multi-pulling method, after a silicon-monocrystal having a dopant concentration within a predetermined range has been pulled, silicon material is recharged into a crucible in a volume corresponding to the reduction in volume of the silicon material in the crucible. After the thus-recharged silicon material has been melt, the pulling of a silicon monocrystal is repeated. This method leads to an improved manufacturing yield. Further, the above-described method makes it possible to produce a plurality of silicon monocrystals from a quartz crucible capable of being usually used only once according to the common Czochralski method. Consequently, the cost of the crucible is decreased, thereby resulting in a decrease in manufacturing costs of the silicon monocrystal.
In addition to the multi-pulling method, there is a continuous Czochralski method (a CCZ method). According to this method, during the course of manufacture of a silicon monocrystal, a silicon monocrystal is pulled while silicon material is supplied to the silicon melt within the crucible which has been reduced in volume as a result of pulling of the silicon monocrystal. This method makes it possible to pull a silicon monocrystal while the quantity of silicon melt is maintained. Therefore, the concentration of oxygen in a silicon monocrystal and the resistivity of the silicon monocrystal, both of which depend on the quantity of silicon melt, can be made constant. Further, the method permits pulling of a long silicon monocrystal, which in turn makes it possible to considerably improve a manufacturing yield of the silicon monocrystal and to reduce manufacturing costs of the silicon monocrystal.
The previously-described multi-pulling method and CCZ method require recharge of silicon material to the crucible containing silicon melt after a silicon monocrystal has been pulled or while the silicon monocrystal is being pulled.
As silicon material recharging methods, there are a method of recharging granular silicon material and a method of recharging silicon material in the form of a rod or block, as shown in FIG. 1. In the method shown in FIG. 1, a large amount of silicon material 55 in the form of a rod or block is recharged into a crucible 51 containing residual silicon melt 52 at one time after a silicon monocrystal 53 has been pulled from the silicon melt 52.
The weight of silicon material to be recharged is approximately 80 kg in case of pulling a 200 mm diameter crystal and approximately 200 kg in case of pulling a 300 mm diameter crystal, and as the period of time required to recharge such a silicon material is shorter, the time required to manufacture a silicon monocrystal can be reduced, whereby the productivity of a silicon monocrystal can be improved. For this reason, a larger recharge rate of silicon material is desirable. If silicon material is recharged to the silicon melt while it is in the form of a rod or block, the recharge rate will become larger. However, intensive heating becomes necessary to melt a large quantity of silicon material recharged in a short period of time. A quartz crucible is apt to become damaged by the intensive heating. Consequently, dislocations may generate easily during the course of growth of a monocrystal. In the case where granular silicon materials are recharged to a silicon melt, the recharge rate can be increased; however, intensive heating becomes necessary, thereby causing the same problems as previously described.
Another method as shown in FIG. 2 is also known (see Japanese Patent Application Laid-Open (kokai) No. 62-260791). In the method shown in FIG. 2, the surface of silicon melt 62 remaining in a crucible 61 after a silicon monocrystal has been pulled is solidified, and silicon material 64 is deposited on the thus-solidified surface 63 of the silicon melt 62 through a feed pipe 60.
According to this method, silicon material is melted after having been deposited on the solidified surface of silicon melt. Therefore, in the case where a particularly large amount of silicon material is recharged, or the case where silicon material is supplied in such a highly-dense granular form, intensive heating becomes necessary to melt the silicon material. As a result, as in the previously described conventional method, the quartz crucible becomes apt to be damaged, thereby resulting in dislocations being apt to occur during the course of growth of a monocrystal.
The inventors of the present invention have previously proposed a method of feeding granular silicon material to silicon melt (Japanese Patent Application Laid-Open (kokai) No. 6-286246). In this method, granular silicon material is fed to silicon melt through a feed pipe from a feeder. Granular silicon material is first caused to stagnate in the feed pipe. At this time, the granular silicon material discharged from the tip end of the feed pipe is deposited on the silicon melt. Subsequently, the tip end of the feed pipe is brought into contact with the thus-deposited granular silicon material, whereby the granular silicon is fed to the silicon melt while the stagnation of the granular silicon material in the feed pipe is maintained.
According to this method, a high speed at which the granular silicon material falls on the silicon melt from the feeder through the feed pipe can be changed to a slow speed at which the granular silicon material stagnated in the feed pipe is discharged to the silicon melt from the tip end of the feed pipe, without changing a feed rate of the granular silicon material to the silicon melt. Consequently, scattering of the granular silicon material and splashing of the silicon melt can be prevented.
In this method, it is important to maintain the granular silicon material stagnated in the feed pipe. It is possible to maintain the granular silicon material stagnated in the feed pipe by increasing the feed rate of the granular silicon material from the feeder to the feed pipe to be maintained so as to become equal to or higher than the discharge rate of the granular silicon material from the feed pipe to the silicon melt.
However, the discharge rate of the granular silicon material from the feed pipe to the silicon melt varies according to the state of an internal wall of the feed pipe, the structure and diameter of the tip end of the feed pipe, a distribution of particle size of the granular silicon material, and the state of the surface of the granular silicon material. However, the discharge rate of the granular silicon material may significantly change as a result of sudden collapse of the silicon material stagnated on the silicon melt, and, in this event, it is difficult to match the feed rate of the granular silicon material from the feeder to the feed pipe to this discharge rate in a superiorly responsive manner. Therefore, there is a risk of the granular silicon material stagnated in the feed pipe momentarily disappearing as a result of flowing into the silicon melt. To prevent such a risk, the maximum discharge rate of the granular silicon material from the feed pipe to the silicon melt is previously estimated, and the granular silicon material is fed from the feeder to the feed pipe at a rate equal to or higher than the maximum discharge rate.
In the case where the feeder is made of common metal such as SUS (stainless steel), in order to prevent the metal from being mixed into the silicon melt as a result of the granular silicon material from coming into direct contact with the metal, the internal surface of the feeder is coated or lined with material such as fluororesin that is hardly contaminated by heavy metal. However, the coating or lining is susceptible to abrasion if the feed rate of the granular silicon material from the feeder to the feed pipe is large.
In the case where a feeder formed from quartz or silicon is used, there arise no problems even if the granular silicon material comes into direct contact with the feeder. The internal surface of the feeder is covered with neither a coating nor a lining, and therefore the foregoing problem will not arise. However, the feeder made of quartz or silicon is relatively fragile. For this reason, if the feed rate of the granular silicon material from the feeder to the feed pipe is high, vibrations of the feeder must be increased, thereby resulting in fracture of the feeder.
In view of the previously-described problems, a feed rate of the granular silicon material from the feeder to the feed pipe is desired to be as low as possible.